First DIY Project - ADM 1540 EQ rack job - Where to get bits

[Chinagraf]

Hi Gary, yes the different resistor are necessary, because the direct in/capacitor out vs transformers would have different impedances, so they need slightly different values. 1/4 watt resistors should be fine, you could get away with something like 10% tolerance ones but as your only doing a few probably worth seeing if you can find something like metal film ones that are tighter specced, even 1% if you can find them in the value you need. The only critical one is marked as 1% tolerance, but you aren't using it as it's for the -200.

[rob]

hard to say without seeing the full documentation, But on my ADM schematic there are resistors and caps variations for the transformer and transformerless versions. These do more than just change impedance, they change the whole front and back end circuits.

[Paul Maybury]

Hi Rob, that's very interesting, do the changes effect gain through the circuit?
Cinemag suggested I try this transformer? It will at least fit, and the phase, bandwidth and distortion specs seem good.
http://cinemag.biz/misc_trans/PDF/CM-2436.pdf
Cheers,
Paul

[Paul Maybury]

P.S they can be ordered without the Mu Metal shield to make them cheaper.
Would the modules own shielding be enough?
Cheers,
Paul

[JLM Audio]

Info on how to mod ADM 1540 EQ Ver 2 (NO input and output transformer fitted) to electronic balanced in with just a couple of resistors can be found on the JLM Audio forum at the link below.

ADM 1540 EQ MOD INPUT to BALANCED IN

[Hookemeister]

Hi there 1540 owners,

well Joe's provided you with a balanced mod input in the previous post so here's the mod for adding a transformer output.

ADM 1540 Transformer Mod.JPG

ADM 1540 Transformer Mod 2.JPG

If you've got the ADM O/P transformer this is a simple mod.

Let me know if you need the wiring for a non ADM transformer.

Personally if I was doing the mod I'd mount the transformer externally and have the output switchable between capacitor coupled output or transformer output... just adds a bit more flavour.

Hope this helps

[JLM Audio]

WARNING If wanting to fit an original internal ADM output transformer or DC couple a output transformer as shown in the above post by Greg you must check you have no dc offset at the output opamp first. Several of the old ADM 1540 we have tested have to much DC offset on the output opamp to direct connect an output transformer due to ageing of several parts.

I would always leave the output capacitor coupling in unless fitting the internal output transformer as this will protect the output transformer from DC magnetising or open circuit failure and destroying the output opamp if a power rail is lost.

If you can hear a difference between the DC coupled and capacitor coupled outs then you either have DC offset or the 2 x 500uF caps need changing as there ESR is now to high and/or they are leaking as well.

[Paul Maybury]

Thanks Joe and Greg, this is great info!
As far as my searching tells me, this is the only place on the net that this is available so concisely .
Looks like I will be trying Joe's active balanced input mod, and fitting ADM output transformers (I have these already) as per Greg's directions.
Joe, if there is too much DC at the output, should I be looking at re capping the modules, checking resister values or swapping out I.C's? Or is this something for an actual tech rather than a hack like me?
Cheers,
Paul

[ishumusic]

Yes, loving all this info. Thanks guys.

[JLM Audio]

Thanks Joe and Greg, this is great info!
Joe, if there is too much DC at the output, should I be looking at re capping the modules, checking resister values or swapping out I.C's? Or is this something for an actual tech rather than a hack like me?
Cheers,
Paul

The DC offset seems to be caused by some of the aged LF356 opamps but can be usually stopped if in any of the early opamp stages by replacing all the 47uF 25v caps. We change these caps to 100uF 50v Non Polar. Watch the DC at the output of the output opamp while switching EQ in and out. And while EQ in switch each band to full boost and cut to see if offset is ok or at least stable at all settings. If offset is stable see the ADM 1540 circuit for the add on R82 trimpot and R81 offset fix mod.

We are going to do a pair of Seans 1540 with different opamps in the low end to see if we can fix the wooden thwack sounding low end these eq's have. As we have done this before in WBS EQ's with great success.